In telecommunication applications, light such as generated from a laser diode, LED or other fibers, is coupled into an optical fiber. The optical fibers could be single or multimode fibers. Problems in coupling efficiency develop when trying to couple short wavelength infrared light from an InGaAsP laser into a single mode fiber.
In one solution, a cleaved fiber is aligned directly to the output facet of a laser. In this simple approach much of light is lost because of the mismatch in the mode size between the laser and the optical fiber. Typically, coupling efficiency in this type of apparatus and method is limited to about 15%. In still another approach, the glass is melted at the tip of the optical fiber to form a spherical lens. The laser light then is pulsed into the formed spherical lens. The spherical surface has aberrations, however, which limits the coupling efficiency to about 50%.
Some optical fibers are produced with hyperbolic lenses, which couple over 70% of the light. A drawback to the use of hyperbolic lenses, however, is the difficulty in achieving, and then maintaining the alignment of the optical fiber to the laser. The tolerance in this alignment is usually about 0.1 microns, a very difficult tolerance to meet in some telecommunication applications. Additionally, the close proximity of the fiber to the laser does not allow other optical devices (e.g. isolators) to be interposed.
A very common solution used presently is the use of high index glass spheres, also referred to as ball lenses.
Ball lenses are inexpensive, easy to align, and therefore desirable, but have low coupling efficiency. A spherical ball lens about 1 mm in diameter can produce about 25% coupling efficiency. Smaller ball lenses can produce higher coupling efficiencies, but the alignment tolerances are reduced proportionately, making adjustments difficult. The symmetry of a ball lens is advantageous nonetheless because all sensitivity to lens tilt is removed. A ball lens is also low in cost, light weight and can be used with a silicon platform on which it can be mounted using glass solders or aluminum oxide bonding methods.
The use of ball lenses is limited in many telecommunication applications, however, because of the spherical aberration of the ball lens. Even with a second ball lens mounted in front of the ball lens on a silicon platform, giving a collimated or nearly collimated beam, the coupling efficiency is still only about 50%. Sometimes this coupling disadvantage is outweighed because the tolerances for aligning the ball lens to the laser are usually sufficient to allow the ball lens alignment to be done by purely mechanical or visual means. Another advantage for two ball lenses is that the two lenses can be spaced to accommodate additional optical elements such as splitters, isolators, wave division multiplexers and other components.
In some applications the residual spherical aberration of the ball lens is overcome by applying a thin polymer label bonded to the ball lens and formed into an aspheric shape. Also, the ball lens can be replaced by a molded glass aspheric lens, or a plano convex gradient index lens. Coupling efficiencies of about 70% have been obtained in these methods, which are more desirable efficiencies in technically complex telecommunication applications.
Again, a drawback with using an aspheric lens is the difficult alignment accuracy required. These lenses must be accurately placed so that a laser is very close to the symmetry axis of the lens. This exacting alignment is very difficult to achieve except by an active method in which the laser is powered, and the focused beam actively monitored while the lens is moved accordingly. Once the lens is set in its proper position, the lens must be secured without motion, often requiring sophisticated, expensive laser welding equipment, and heavy, expensive metallic retainers. These metallic retainers also limit the various degrees of movement freedom sometimes required in alignment. Also, the laser and the lens are usually attached to a common metal base to assure stability and permit the parts to be laser welded.
Not only is this system expensive, but the metal parts add weight to the assembly and may reduce the shock resistance of the final package if the assembly has to be placed on a thermoelectric cooler. There is also the additional thermal mass, and the proximity of the laser to metal components could have detrimental effects on the radio frequency characteristics of the device if high speed modulation is required.
It is therefore desirable if a ball lens and similar optics could be used in a laser application, but with increased coupling efficiencies comparable to the designs used in aspheric lenses.